The light weight metals of aluminum and magnesium have very large markets for they are utilized in a wide variety of industries. In a lesser way, titanium is also utilized as a light weight fabrication material. These metals suffer from some drawbacks, however, which limit their usefulness. These include low stiffness (low modulus of elasticity), high thermal coefficient of expansion, and low strength. Some of these drawbacks have been overcome through the use of metal matrix composites of these metals. Typically, metal matrix composites are composed by adding ceramics to the metals. The primary objectives of these ceramic additives have been to increase the modulus of elasticity and to reduce the thermal coefficient of expansion. When fibrous material, such as silicon carbide whiskers, are added, strengthening has been observed. Other added materials include the fibers of boron metal, carbon, aluminum silicate, and aluminum oxide, Still other typical strengthening agents are aluminum oxide particulates, boron carbide and silicon carbide in various forms.
Of these, only aluminum oxide particulate and silicon carbide particulate have been extensively utilized in the aluminum-based matrix. To add either of these to molten aluminum, a continuous stirring action must be utilized because the specific gravity of the additives are significantly greater than the molten aluminum. This means that constant agitation of the aluminum/additive mixture is required to keep the additive in substantially uniform distribution throughout the molten aluminum. Similar problems are encountered for mixtures of the same additive with molten magnesium. Stirring the molten metal can keep the additive distributed throughout the molten metal, but such continuous stirring causes oxide inclusions and hydrogen to contaminate the melts.
Furthermore, because of the contamination and the non-uniform nature of the metal matrix composites, remelting (for recycle, etc.) is a problem due to the variability of the resulting feed product.
In the prior art, various methods and compositions have been developed to overcome these difficulties. In some instances, powdered metal processes have been used to make metal matrix composite materials. For example, U.S. Pat. No. 5,573,607 describes a metal matrix composite wherein particles of silicon boride are combined with aluminum, magnesium or titanium to form a metal matrix composite. According to the process therein described, the silicon boride particles can be either pre-blended with the metal particles or stirred into the melt to form the metal matrix. Other examples of the use of powder metallurgy for the manufacture of metal matrix composite materials are shown and described in U.S. Pat. No. 5,712,014 which describes the use of boron carbide in the preparation of the metal matrix composite; and in U.S. Pat. No. 5,948,495 which describes the use of powder metallurgical techniques to make an aluminum/ceramic disk substrate.